Magnetrons are used in linear accelerator systems (linacs) to generate X-rays, and one use of such linacs is to generate X-rays for the treatment of tumours in radiotherapy. In an attempt to deliver the optimum dose of radiation to a tumour, linacs are being mounted on gantries which rotate around the patient, sometimes at high speed, while the X-ray dose is being delivered. This occasionally causes a problem, in that to achieve optimum performance the cathode must be held in a precise position in a hollow cylindrical anode with a high voltage between the anode and cathode. The cathode may be supported on a pair of electrically conducting arms which are anchored into the vacuum envelope at their ends.
Thus, referring to FIGS. 1 and 1a, which are, respectively, a schematic fragmentary axial cross-section through a radially-extending portion of a known magnetron and a fragmentary end view of the interior of the anode, the cathode 1 is supported in a hollow cylindrical anode 2 by means of tungsten support arms 3, 4. The radially-extending portion of the vacuum envelope, generally termed a sidearm, is indicated generally by the reference numeral 5, and carries on its exterior the cathode terminals 6, 7 across which a DC heater voltage for the cathode is applied, superimposed on the high negative voltage required for operation of the magnetron. The main body 8 of the magnetron is made of metal, and has channels 9, 10 to accommodate the support arms 3, 4. The radially-extending portion includes a metal ring 11, which is welded to the main body 8, and glass thimble 12 which is bonded to the metal ring. The support arms 3, 4 are secured in a vacuum-tight fashion in apertures in the dished upper end 13 (as seen in the drawing) of the thimble, and form the cathode terminals 6, 7. At the lower end, as seen in FIG. 1, the cathode support arms are connected to opposite ends of the cathode 1 by means of leads 16, 17. The cathode support arms 3, 4 terminate short of the cylindrical anode space 2, to allow room for the cathode to be inserted in an axial direction during manufacture (see FIG. 1a), and the leads are only connected when the cathode has been assembled into the anode space. The lead 16 at one end may be v-shaped, the apex being connected to the cathode support arm 3, and the ends of the limbs being connected to the cathode. The lead 17 at the other end may be a conductor bent into parallel strands and connected to a heater lead extending from the other end of the cathode through an insulating collar (not shown, but illustrated in our US patent publication no. 2009/0236991).
It is believed that in some instances, the support arms 3, 4 pick up mechanical vibrations, which can impair the correct functioning of the magnetron.